Generally, a pump includes a motor part having a stator generating a magnetic field and a controller controlling the stator; a pump part having an impeller driven by the magnetic field generated by the stator to suck and discharge a liquid such as water; and a partition member isolating the motor part from the pump part.
The pump part increases the pressure of the sucked liquid to discharge same by the impeller. In case of a centrifugal pump, the impeller has a plurality of blades fixed thereto, the whole body of each blade being curved backward with respect to a rotational direction to reduce loads applied thereto.
Since, however, the pressure in the centrifugal pump is increased by a centrifugal force, the rotational speed needs to be increased in order to discharge the liquid with a higher pressure by using a small pump. For this reason, when a gas-laden liquid is sucked, there occurs a problem that the liquid and the gas are separated by the strong centrifugal force applied thereto, and the gas having a smaller specific gravity than the liquid stagnates around a central part of the impeller, thereby decreasing the performance of the pump.
To solve the problem, a pump having a guide member projecting from a pump case towards the impeller has been proposed (see, for example, Japanese Patent Laid-open Application No. 2001-234894).
By employing such a pump case, the gas bubbles ladened in the liquid are disaggregated by the portion of the guide member disposed at a central part of the impeller and discharged through a discharge port, thereby preventing the gas from stagnating in the impeller.
However, if a pumping rate is small and the gas is admixed into the liquid, the flow of the liquid becomes less. In such a case, it is difficult to guide the disaggregated gas bubbles to the discharge port disposed at an outer periphery of the impeller, even with the scheme disclosed in the Patent Application supra.
If a central part of a portion of the liquid discharged by the impeller is fed back into the impeller through a reflux passage for example, it may be possible to discharge the gas stagnant at the central part of the impeller. However, in an exterior rotor structure in which a stator is installed inside the rotor as in the Patent Application supra, it is not possible to feed a sufficient amount of liquid back into the central part of the impeller, so that it is difficult to discharge the gas continuously introduced by being laden in the liquid.